The present invention relates to UV screening compositions, methods for their preparation and their use. The invention in particular relates to, for example, compositions comprising particulate oxides, their preparation and their use as, for example, paints, plastics, coatings, pigments, dyes and compositions for topical application, in particular, for example, sunscreens.
The effects associated with exposure to sunlight are well known. For example, painted surfaces may become discoloured and exposure of skin to UVA and UVB light may result in, for example, sunburn, premature ageing and skin cancer.
Commercial sunscreens generally contain components which are able to reflect and/or absorb UV light. These components include, for example, inorganic oxides such as zinc oxide and titanium dioxide.
Titanium dioxide in sunscreens is generally formulated as “micronised” or “ultrafine” (20-50 nm) particles (so-called microreflectors) because they scatter light according to Rayleigh's Law, whereby the intensity of scattered light is inversely proportional to the fourth power of the wavelength. Consequently, they scatter UVB light (with a wavelength of from 290 to 320 nm) and UVA light (with a wavelength of from 320 to 400 nm) more than the longer, visible wavelengths, preventing sunburn whilst remaining invisible on the skin.
However, titanium dioxide also absorbs UV light efficiently, catalysing the formation of superoxide and hydroxyl radicals which may initiate oxidations. The crystalline forms of TiO2, anatase and rutile, are semiconductors with band gap energies of about 3.23 and 3.06 eV respectively, corresponding to light of about 385 nm and 400 nm (1 eV corresponds to 8066 cm−1).
An incident photon is absorbed by titanium dioxide if its energy is greater than the semiconductor band gap Eg shown in FIG. 1. As a result an electron from the valence band (vb) is promoted into the conduction band (cb) (transition [1]). If the energy of the incident photon is less than Eg it will not be absorbed as this would require that the electron be promoted to within the band gap and this energy state is forbidden. Once promoted, the electron relaxes to the bottom of the conduction band (transition [2]) with the excess energy being emitted as heat to the crystal lattice.
When the electron is promoted it leaves behind a hole which acts as a positive particle in the valence band. Both the electron and the hole are then free to migrate around the titanium dioxide particle. The electron and hole may recombine emitting a photon of energy equal to the band gap energy. However, the lifetime of the electron/hole pair is quite long due to the specific nature of the electronic band structure. Thus there is sufficient time (ca. 10−11 s) for the electron and hole to migrate to the surface and react with absorbed species.
In aqueous environments, the electrons react with oxygen, and the holes with hydroxyl ions or water, forming superoxide and hydroxyl radicals:TiO2+hυ→TiO2(e−/h+)→e−(cb)+h+(vb)e−(cb)+O2→O2.−→HO.2 h+(vb)+OH.→.OH
This has been studied extensively in connection with total oxidation of environmental pollutants, especially with anatase, the more active form [A. Sclafani et al., J.Phys. Chem., (1996), 100, 13655-13661].
It has been proposed that such photo-oxidations may explain the ability of illuminated titanium dioxide to attack biological molecules. Sunscreen titanium dioxide particles are often coated with compounds such as alumina, silica and zirconia which form hydrated oxides which can capture hydroxyl radicals and may therefore reduce surface reactions. However, some TiO2/Al2O3 and TiO2/SiO2 preparations exhibit enhanced activity [C. Anderson et al., J. Phys. Chem., (1997), 101, 2611-2616].
As titanium dioxide may enter human cells, the ability of illuminated titanium dioxide to cause DNA damage has also recently been a matter of investigation. It has been shown that particulate titanium dioxide as extracted from sunscreens and pure zinc oxide will, when exposed to illumination by a solar simulator, give rise to DNA damage both in vitro and in human cells [R. Dunford et al, FEBS Lett., (1997), 418, 87-90].
The present invention provides UV screening compositions which address the problems described above and are less liable to produce DNA damage on illumination than conventional sunscreen compositions.